12,324 research outputs found
Pair production in a strong electric field: an initial value problem in quantum field theory
We review recent achievements in the solution of the initial-value problem
for quantum back-reaction in scalar and spinor QED. The problem is formulated
and solved in the semiclassical mean-field approximation for a homogeneous,
time-dependent electric field. Our primary motivation in examining
back-reaction has to do with applications to theoretical models of production
of the quark-gluon plasma, though we here address practicable solutions for
back-reaction in general. We review the application of the method of adiabatic
regularization to the Klein-Gordon and Dirac fields in order to renormalize the
expectation value of the current and derive a finite coupled set of ordinary
differential equations for the time evolution of the system. Three time scales
are involved in the problem and therefore caution is needed to achieve
numerical stability for this system. Several physical features, like plasma
oscillations and plateaus in the current, appear in the solution. From the
plateau of the electric current one can estimate the number of pairs before the
onset of plasma oscillations, while the plasma oscillations themselves yield
the number of particles from the plasma frequency.
We compare the field-theory solution to a simple model based on a
relativistic Boltzmann-Vlasov equation, with a particle production source term
inferred from the Schwinger particle creation rate and a Pauli-blocking (or
Bose-enhancement) factor. This model reproduces very well the time behavior of
the electric field and the creation rate of charged pairs of the semiclassical
calculation. It therefore provides a simple intuitive understanding of the
nature of the solution since nearly all the physical features can be expressed
in terms of the classical distribution function.Comment: Old paper, already published, but in an obscure journa
Photodisintegration of light nuclei for testing a correlated realistic interaction in the continuum
An exact calculation of the photodisintegration cross section of 3H, 3He and
4He is performed using as interaction the correlated Argonne V18 potential,
constructed within the Unitary Correlation Operator Method (VUCOM).
Calculations are carried out using the
Lorentz Integral Transform method in conjunction with an hyperspherical
harmonics basis expansion. A comparison with other realistic potentials and
with available experimental data is discussed. The VUCOM potential leads to a
very similar description of the cross section as the Argonne V18 interaction
with the inclusion of the Urbana IX three-body force for photon energies 45< w
< 120 MeV, while larger differences are found close to threshold.Comment: 9 pages, 6 figure
Breaking of N=8 magicity in 13Be
Structure of Be was investigated with antisymmetrized molecular
dynamics. The variation after spin and parity projections was performed. An
unnatural parity state was suggested to be lower than state
indicating that vanishing of the N=8 magic number occurs in Be. A
low-lying state with a configuration was also suggested.
Developed cluster structures were found in the intruder states. Lowering
mechanism of the intruder states was discussed in terms of molecular orbitals
around a core.Comment: 9 pages, 8 figures, submitted to PR
Ions in Fluctuating Channels: Transistors Alive
Ion channels are proteins with a hole down the middle embedded in cell
membranes. Membranes form insulating structures and the channels through them
allow and control the movement of charged particles, spherical ions, mostly
Na+, K+, Ca++, and Cl-. Membranes contain hundreds or thousands of types of
channels, fluctuating between open conducting, and closed insulating states.
Channels control an enormous range of biological function by opening and
closing in response to specific stimuli using mechanisms that are not yet
understood in physical language. Open channels conduct current of charged
particles following laws of Brownian movement of charged spheres rather like
the laws of electrodiffusion of quasi-particles in semiconductors. Open
channels select between similar ions using a combination of electrostatic and
'crowded charge' (Lennard-Jones) forces. The specific location of atoms and the
exact atomic structure of the channel protein seems much less important than
certain properties of the structure, namely the volume accessible to ions and
the effective density of fixed and polarization charge. There is no sign of
other chemical effects like delocalization of electron orbitals between ions
and the channel protein. Channels play a role in biology as important as
transistors in computers, and they use rather similar physics to perform part
of that role. Understanding their fluctuations awaits physical insight into the
source of the variance and mathematical analysis of the coupling of the
fluctuations to the other components and forces of the system.Comment: Revised version of earlier submission, as invited, refereed, and
published by journa
Pile-in-Tube Foundations with Reserve Switch-off Elements and Other Systems for Seismic Response Adaptive Control
Predominant ground motion frequencies and spectra could differ sufficiently at different earthquakes on one site. The reasons of the differences are the epicenter distance, the focal depth, and many others. As it is well known, the frequency spectral configuration and predominant frequency values influence the structural seismic response and seismic behavior of structures (Mexico earthquakes 1957, 1962; Bucharest, 1977, Spitak, Armenia, 1988 and other). The maximum seismic response depends of how close the fundamental frequency values of the structure are to the predominant ground motion frequencies. Seismoisolation as an effective approach to seismic response control became popular recent decades in many countries. The conclusions of RCEE analytical and experimental studies are that in abovementioned cases when different spectra and predominant frequencies could be expected at a given site structures with changing (self-adjusting) natural frequencies could be effective for adaptive seismic response control. Several dozens of structures with switch-off reserve elements are designed with RCEE participation and constructed in Siberia, in Caucasus, near Lake Baykal and at other earthquake dangerous areas of Russian Federation. Besides, of pile-in-tube foundations also other structural systems are constructed. Among these systems are rocking supports with reserve switch-off elements, flexible columns with switch-off reserve rigid elements,pile-in-tube, sliding supports and others
Preliminary study of advanced turboprop and turboshaft engines for light aircraft
The effects of engine configuration, advanced component technology, compressor pressure ratio and turbine rotor-inlet temperature on such figures of merit as vehicle gross weight, mission fuel, aircraft acquisition cost, operating, cost and life cycle cost are determined for three fixed- and two rotary-wing aircraft. Compared with a current production turboprop, an advanced technology (1988) engine results in a 23 percent decrease in specific fuel consumption. Depending on the figure of merit and the mission, turbine engine cost reductions required to achieve aircraft cost parity with a current spark ignition reciprocating (SIR) engine vary from 0 to 60 percent and from 6 to 74 percent with a hypothetical advanced SIR engine. Compared with a hypothetical turboshaft using currently available technology (1978), an advanced technology (1988) engine installed in a light twin-engine helicopter results in a 16 percent reduction in mission fuel and about 11 percent in most of the other figures of merit
Potential of liquid-methane fuel for Mach-3 commercial supersonic transports
Liquid methane fuel for commercial fixed arrow wing supersonic transport
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